The present invention relates to an LED device used mainly for backlight of liquid crystal display of personal digital assistant and so on and for all kinds of indicators. Particularly, the LED device has phosphors that are excited by light emitted from an LED to emit light of different wavelength from the light emitted from the LED and is used for an LED light source emitting white or neutral light.
Recently, demand for cellular phone and so on using color LCD has been expanded. As a backlight source of the color LCD, a white LED device has been used. A white LED device 100 as shown in FIG. 14 is provided with a base 101 made of white-light reflecting resin, i.e., AMODEL®, VECTRA® and so on and metal frames 102. On the inside bottom of a mortar-like or cone-like portion of the base 101 and on the metal frame 102, an LED chip (light emitting diode) 103 is mounted. The LED chip 103 is a blue LED emitting light having a wavelength of around 460 nm. Conduction of the LED chip 103 is made through an Au wire 104 and conductive adhesive 105. According to the structure of the LED chip 103, the conduction may be accomplished through the Au—Au connection by a facedown method and also may be made through two Au wires. The LED chip 103 is fixed by a transmission type of resin 106 such as epoxy resin, silicon resin and so on.
In order to obtain predetermined light tone and chromaticity coordinate, the resin 106 includes YAG phosphors 107 which emit light of different wavelength from the wavelength of light emitted from the LED chip 103, i.e., which absorb a part of the light emitted from the LED chip 103 and convert the wavelength of it to emit light. As described above, in the generally used white LED device 100, a pseudo-white light emission is made by a combination of the blue LED chip 103 and the YAG phosphors 107. That is, the pseudo-white light emission is accomplished through a color mixture emission by a combination of or a complementary color of a blue light emitted from the blue LED chip 103 and a yellow light emitted from the excited YAG phosphors 107 (for example, refer to Japanese Laid-open patent publication No. 2000-223750, FIG. 2).
However, the pseudo-white light is not obtained by a color mixture of three primary colors, i.e., red, green and blue, resulting in a disadvantage of less reproducibility of especially red. Thus, a white LED device (not shown) of good color quality which is a combination of a blue LED 103 and phosphors emitting red, blue and green lights can be devised. As the phosphors emitting red, blue and green lights have a low excitation efficiency or a low wavelength-conversion efficiency, luminance of the white LED device is low, resulting in a disadvantage of being off from practical use.
In order to solve the above problems, in stead of the blue LED 103 emitting light having a blue-region wavelength of 460 nm, it can be devised to use an LED emitting light having a short blue-violet-region wavelength of 430 nm or below to improve the excitation efficiency of the phosphors. However, when the wavelength of the emitting light is changed to an ultraviolet region from the blue-violet-region wavelength of 430 nm or below, even the high-efficiency light-reflecting resin (AMODEL®, VECTRA® and so on) used as the base 101 of the LED device 100 of the visible light region has a rapidly reduced light reflectance in a short wavelength region. Thus, the reflection on the base 101 (the reflection on the mortar-like inner wall surface of the base 101) can not be conducted, causing a reduction of luminance of LED device 100. FIG. 10 shows reflectance of AMODEL® A-4122NL used for the base 101.